LED (light-emitting diode) light sources have been paid great attention as light sources taking the place of incandescent lamps, fluorescent lamps and the like in recent years. For example, JP Pub. No. 2012-244737 (hereinafter referred to as “Document 1”) discloses an LED power supply for energizing LED(s) as an illumination light source.
The LED power supply described in Document 1 includes a rectifier, a ripple filter circuit, a power converter circuit and the like. The rectifier is configured to convert (full-wave rectify) an alternating current (an alternating voltage) to a direct current (a direct voltage), where the alternating current is supplied from a commercial AC power supply. The ripple filter circuit is configured to remove ripples from (smooth) a pulsating current (a pulsating voltage) as a result of full-wave rectification by the rectifier. The power converter circuit is configured to convert (step down) the direct current (the direct voltage) as a result of smooth by the ripple filter circuit into a DC current (a DC voltage) corresponding to LED(s) as a load.
The LED power supply further includes a stop control circuit and a mask circuit. The stop control circuit is configured to stop the power converter circuit when the load (LED(s)) is removed during an operation of the power converter circuit, namely in no-load condition. Specifically, the stop control circuit is configured to detect a DC current (a load current) output from the power converter circuit, and to detect presence of the no-load condition if the load current is lower than a predetermined lower limit, thereby to stop the power converter circuit.
The mask circuit is configured to prohibit the stop control circuit from operating if a value of the DC voltage smoothed by the ripple filter circuit is higher than a predetermined value, and to permit the stop control circuit to operate if the value of the DC voltage is equal to or lower than the predetermined value.
The LED power supply described in Document 1 can therefore suppress abnormal increase in an output voltage thereof in no-load condition to prevent an excessive current from flowing through the load when it is reconnected to a load.
In the LED power supply described in Document 1, there is however a problem that wrong determination (a malfunction) is liable to occur when an input voltage thereof (a power supply voltage from an AC power supply) instantaneously decreases (like momentary power failure or the like) because it detects an input voltage and an output current to judge output prohibition or permission with respect to the power converter circuit from the detection results.